The invention relates to a method for producing light metal castings, in particular cylinder heads, cylinder blocks and/or crankcases for internal combustion engines, with a sand mold for forming a molding cavity for the casting, which comprises outer mold parts, at least one core and at least one feeder for forming a riser, wherein the mold is provided with an in-gate for the metal melt and wherein the metal melt is filled into the molding cavity of the mold under the effect of gravity.
It is a known practice to use sand molds and permanent molds, so-called chill molds, for the production of light metal castings, as well as to use molds that are composed of permanent mold parts, so-called half-chills, and sand-casting mold parts. On the one hand, the main advantage of the series production of castings with the sand mold lies in this case in the high production speed for producing the sand molds and, on the other hand, in the high production speed for the casting with respect to the casting rate. In contrast to the casting with chill-molds, it is not necessary after each casting operation to wait for the mold to cool down before reusing it. Since it is also possible to recycle the sand used for producing the outer mold parts and the core, light metal castings such as cylinder heads for internal combustion engines, are nowadays cast at high productivity in sand molds. In this case, the metal melt is always poured into the sand mold under the effect of gravity, for the most part with the so-called gravity casting. To improve the texture, feeders are used at the mold, which contain a feeder material that matches the material of the casting for high-quality castings and functions as a so-called riser. For this, the somewhat coarser texture as well as the possibly occurring micro-porosity when casting in sand-based molds as compared to the casting in chill molds is accepted.
Furthermore a known method of low-pressure casting of light metal melts in permanent chill molds, wherein a fine, dense texture can be achieved by applying pressure with a gas to the melt inside the chill mold is disclosed in DE-A-21 33 421; and DE-A-28 18 442. The application of pressure is no problem because of the stability of the chill mold. However, since the chill molds have a very long cooling-down period, an extremely high number of very expensive chill molds are needed for high production speeds. Owing to the frequently occurring changes in the castings, a high number of chill molds must therefore be changed, or new ones must be produced.
From the DE-B-11 35 619 publication it is known to use a standard sand mold with a fixed upper frame and a fixed lower frame, wherein for the production of metal castings with a dense structure, the sand mold consisting of upper and lower frames is covered with a sealed dome or the upper and lower frame of the mold are connected such that they are sealed and a dome that is also sealed is placed over the free surface of the upper frame after the molding cavity of the mold has been filled with metal melt. The inside space of the dome is admitted with a pressurized gas, which then acts upon and compresses the melt via the porosity of the mold sand, in particular via the free melt surfaces in the region of the feeders and risers. This method is time-consuming despite the use of sand molds since either a complete dome must be placed in each case over the sand mold or care must be taken during the fitting together of the sand mold so that lower frame and upper frame as well as the pressure dome that is fitted on top are sealed completely when connected.
From U.S. Pat. No. 2,960,736 it is known that in order to solve the sealing problems, a pressure dome is fitted over the feeder opening, which is provided with a sealing collar that dips into the melt inside the feeder. The disadvantage of this system is, among other things, that a relatively large melt surface must exist in the end section of the feeder for sealing purposes, independent of the size of the castings to be cast, so that following the separation of the feeders from the finished casting a correspondingly large amount of material must be melted on again.